Installation vessels are used to install items during the setup of a hydrocarbon production system that produces hydrocarbons from the sea floor, a system where hydrocarbons are transferred to or from a shore-based installation, and other offshore systems especially for hydrocarbon transfer and well maintenance. These include installations where a floating body is held by chains extending from a turret or by spread mooring. The installation vessel installs items on the sea floor, including anchors, piles, manifolds, subsea trees (wellheads), templates and pumps, items that are to lie at a height between the sea floor and the sea surface such as buoyancy tanks that are to be attached to risers, and other in-sea items or tools including cables, chains, and underwater hammers. A conventional installation vessel has a single work deck extending from the stern of the vessel to about halfway to the bow. The limited space on the work deck limits the amount of materials, equipment and tools that can be stowed. The space is limited especially because space must be left between winches and a stern roller, between which elongated elements such as cables, chains and hoses are rapidly moved into or out of the sea. The rapidly moved elongated elements create a danger to personnel working on the deck.
Considerable material is usually placed at a mobilization harbor that is close to the installation site. A conventional installation vessel sails a long distance to a mobilization harbor that is located near where a system is to be installed, maintained, etc. The installation vessel has to interrupt the installation work one or more times during an installation, to sail to the mobilization harbor to pick up materials and equipment that could not be taken on board earlier because of the limited storage space on the vessel. Only after the loading and sailing back is completed, can the installation work be continued. Another ship cannot perform the transportation, because this would require the transfer of the material and equipment to the installation vessel at sea, which is normally too risky.
The above-described problems are especially relevant for installations in deep waters, which are normally located much further offshore and therefore at larger distances from the mobilization harbor. Also, the installation packages that include equipment and modules to be installed in deep water are usually much larger in size and weight than for waters of moderate depth.